1. Field of the Invention
The instant invention is directed to an apparatus for continuous filtration and flow of molten thermoplastic polymer. More specifically, the instant invention is directed to an apparatus for continuous filtration and flow of molten thermoplastic polymer from a melt source to a shaping apparatus.
2. Background of the Prior Art
The employment of molten thermoplastic polymers in the production of such large usage applications as man-made fibers, polymeric films and the like represents immense volume activity. In most applications the second processing step in this procedure, subsequent to melt formation, but prior to the shaping of the melt into a fiber, film or the like is the filtration of the melt to eliminate foreign particles, and more importantly, to provide a homogeneous melt. A homogeneous melt assures that gels, impurities or other degradation products formed in the polymerization step are removed. Gels, degradation products and the like, as those skilled in the art are aware, result in the formation of inferior grade polymeric shaped articles.
In a typical thermoplastic melt filtration system two filter means are disposed in parallel configuration. The extruded molten polymer is typically directed into one of two lines by means of a three-way valve or the like. Each of the two lines is provided with a replaceable filter means. When the filter life of the filter means is expired due to plugging etc., the valve is turned and flow is directed to the second branch line equipped with a new filter means. While this second filter means is operating, the first filter means is replaced with a fresh filter and this replacement process is repeated as necessary. In this way, the costly process of shutting down the fiber or film forming process to replace filters is eliminated.
Although this procedure results in a process that eliminates complete shutdown, this process of the prior art has inherent disadvantages which results in substantial loss of product and oftentimes shutdown of production.
To understand the disadvantages of the prior art it is necessary to review the apparatus and process that are currently employed in thermoplastic melt processing. In the prior art, an extruder or the like extrudes a stream of molten thermoplastic polymer to a three-way valve. The three-way valve is provided with three positions -- a closed position, a position providing flow to a first line provided with a filter means and a third position providing flow to a second line also provided with a filter means. The three-way valve also can be set to intermediate positions, permitting flow to both parallel branch lines. In the normal operation of the prior art process, the three-way valve is set to provide thermoplastic melt flow through one of the two lines. In this prior art process the two branch lines, each provided with a replaceable filter means, come together downstream of the filters, meeting to form an inlet of a four-way valve. The outlet of the four-way valve communicates with a conduit in communication with the inlet of a shaping apparatus such as a spinneret, a die or the like where the melt is formed into a shaped article.
In the operation of the prior art process, a filter is replaced by initially moving the upstream three-way valve a slight amount so that some of the melt flow is diverted to the second line although the predominant melt flow is still directed through the first line, the line containing the filter that must be replaced. This results in the filling of the second line. With the parallel disposed first and second lines full with polymer the prior art process calls for the upstream three-way valve to be moved to a position which provided equal melt flow to the first and second lines. At this point the downstream valve is moved for the first time, so that instead of the first line being in communication with the shaping apparatus, and the second line communicating with a vent, communication is made between the second line and the shaping apparatus while the first lines communicate with the vent. Finally, with the downstream valve moved to provide communication with the second line containing the fresh filter, the upstream three-way valve is turned to provide sole communication between the melt source and the second line.
Although the above-described procedure represents the optimum manipulation of flow regimes for the process and apparatus of the prior art, the step in which the downstream valve is moved to provide communication between the on-coming line, having the new filter, and the shaping apparatus presents serious disruption in the normal operation of the shaping operation, i.e., film formation, fiber forming, etc. During the turning of the downstream valve the valve moves through a position in which all flow of the thermoplastic melt to the shaping apparatus is cut-off. As those skilled in the art are aware, a melt pump which moves thermoplastic melt from a melt source to a shaping apparatus is provided with safety controls which automatically cuts off flow when a critical pressure is reached. The momentary cut-off of melt flow to the shaping apparatus often causes a build-up of pressure in excess of the critical pressure resulting in an automatic shutdown of the pump. This results in a situation analagous to the position that was obtained before the introduction of the prior art process. That is, the apparatus must be completely shutdown and restarted, with a resultant serious loss of product and time, whenever a filter change is made.
Although the above result is most common, oftentimes the pump does not cutout so that no shutdown results. However, even this result causes serious problems in that there is still a discontinuity in the polymeric melt flow. Often this causes the formation of bubbles which usually result in a break in the shaped article upon the imposition of a stretching. A break results in a shutdown of production. Even, if no bubbles are formed, that is, in the best possible case, which is not usual, there is a decrease in the melt mass rate of flow resulting in a thinning of the formed sheet or a decrease in the diameter of the formed filament as the case may be. This causes an off-specification product at best or a tear in the shaped article at worse.
The above prior art operation of the process and apparatus for filtration of a thermoplastic melt between a melt source and a shaping apparatus not only results in serious processing difficulties, but moreover, results in inferior shaped article product. This is postulated to be caused by the frequent shutdown of the system which is postulated to cause residual melt in the lines. This melt, due to complex conditions, polymerizes to form gels and the degradation products. Upon restartup of the system the melt flow entrains these gels, many of which ultimately appear in the shaped article. This is especially true if the gels form downstream of the filters in the parallel branch lines.
The above description of the present prior art process indicates a need for a new improved apparatus for filtering and moving a thermoplastic melt between a melt source and a shaping apparatus.